Production of ethanol obtainable using biomass starting materials (bioethanol) is increasing in importance as a means of obtaining renewable liquid fuel, and bioethanol production is being researched not only from edible resources such as starch, but also from lignocellulosic biomass such as wood. When lignocellulosic biomass such as wood is used, the traditional method has been researched to decompose the cellulose in the biomass into monosaccharides with concentrated sulfuric acid and then perform fermentation. However, this method has not been widely adopted because of the difficulty of establishing and managing acid resistance of the apparatuses used to handle the concentrated sulfuric acid, and efficient techniques for recovery of the sulfuric acid.
Other methods have also been investigated, that do not use acids such as sulfuric acid but rather make use of enzymes such as cellulases for monosaccharification of the polysaccharide components of biomass (saccharification), and accomplish ethanol fermentation using yeasts and the like (enzymatic saccharification and fermentation methods). However, when such methods are applied to lignocellulosic biomass such as wood, some sort of treatment (pretreatment) is required before the enzyme saccharification.
The pretreatment for enzyme saccharification is a step in which the cellulose and other materials in the biomass are converted to a state that allows effective action by the enzymes, and it may involve physical or chemical treatment. The physical treatment often involves milling with a ball mill or the like. Chemical treatment involves removal of lignin with chemical agents to obtain cellulose, as in the chemical steps for producing wood pulp. PTL 1, for example, discloses that efficient production of bioethanol can be accomplished by pretreatment of the biomass by alkaline cooking or kraft cooking to remove most of the lignin in the biomass, followed by enzyme saccharification and fermentation.